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<head>
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    <title>Document</title>
    <style>
    .box{
        margin: 10px;
        width: 100px;
        height: 100px;
        background-color: black;
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    .box2{
        /* margin-top: 20px */
        width: 100px;
        height: 100px;
        /* background-color: black; */
        outline: 0;
        padding: 10px;
        border-radius: 1px solid red;
    }
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</head>
<body>
    <div class="container">
        <div class="box"></div>
        <div style="overflow: hidden;">  
            <!-- div并不能创建BFC能力  怎么让它具备就好了
                BFC拥有独立的不受外界影响的行的格式化上下文
                FFC = BFC + Flex
            -->
            <div class="box2"></div>
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